This invention relates to removal of both mercury (Hg) and nitrogen oxides (NOx) from exhaust gas generating from combustion of carbonaceous materials and apparatus for effecting such removal.
Among the 189 substances listed as hazardous air pollutants in the Clean Air Act Amendments (CAAA) of 1990, mercury is a metal species of great concern due to its extreme toxicity and the risk that it can cause to humans and animals if released to the environment. In several countries, legislation is being prepared to limit the emission of mercury to the atmosphere. While most of the trace metals can be efficiently removed in today""s air pollution control system, mercury is present mainly in its vapor phase and is difficult and expensive to remove.
In the US, the EPA maximum achievable control technology (MACT) will limit mercury emissions to 40-110 xcexcg/dscm at 7%O2 for hazardous waste incinerator. For municipal sewage sludge combustor, federal regulation (40CFR Part 61, Subpart E) limits mercury emissions to 3,200 grams per 24 hours.
Nitrogen oxides (NOx) are also an environmental problem because they can initiate reactions resulting in the production of ozone and acid rain. These pollutants can harm forests and lakes, damage buildings and cause health problems. Guidelines for controlling NOx emissions are provided in the 1990 CAAA under the xe2x80x9cNitrogen Oxides Emission Reduction Programxe2x80x9d and xe2x80x9cOzone Non-Attainment Programxe2x80x9d. For municipal sewage sludge combustors, NOx is not regulated at the Federal level yet. However, as a result of the 1990 CAAA, Federal regulation on NOx is anticipated. Consequently, NOx emission from municipal sewage sludge combustors can be limited to the MACT standard, which is no more than the average emissions achieved by the best performing 12% of all operating incinerators. Most state authorities already regulate NOx emissions for their municipal sewage sludge combustors and have very stringent limits.
Of the different technologies available for reducing NOx and Hg, most require additional equipment and the use of expensive and/or hazardous chemicals. Therefore, it would be advantageous to develop a method for removing these compounds utilizing only the standard pollution control equipment and cost efficient non-hazardous chemicals, thereby meeting strict emission limits economically.
A number of different methods have been proposed to reduce mercury and/or NOx emissions from combustor exhaust gas. However, the majority of these processes are more sophisticated due to either the extension of additional equipment or the hazardous nature of the additives. Very few of those methods propose simultaneous reduction of both mercury and nitrogen oxides.
Mercury Removal:
The chemical form of Hg in the gas to be treated is of considerable interest. Ionic mercury is removed with control processes that employ various aqueous scrubbing techniques. Elemental mercury, however, is essentially unaffected by wet scrubbers and requires some type of sorbent or carbon injection process.
Mercury typically can be removed from the combustor exhaust gas in two ways, (1) adsorption via sorbent injection into the exhaust gas or via flow through fixed sorbent bed at low temperature upstream of a particulate matter collector, and (2) wet scrubbing with conversion of the elemental mercury into a more soluble species that can be easily absorbed in a scrubber.
WO 9,517,240 describes a method for improving mercury removal capability of a flue gas purification system by introducing sulfur vapors into the flue gas stream where admixed flue gases and sulfur vapors contact solid particulate (calcium hydroxide) materials in the flue gas. Calcium hydroxide adsorbs mercury and sulfur vapors and catalyzes reactions forming solid products comprising mercury. The solid products comprising mercury are separated, thereby forming a purified flue gas stream. The solid particulate materials are formed in situ by reaction in a spray dryer between an aqueous dispersion of calcium hydroxide and the acidic materials in the flue gas at a temperature between 70 and 170xc2x0 C.
U.S. Pat. No. 4,889,698 discloses a process in which powdery activated carbon is injected immediately before, during or after an alkali reagent (limestone or sodium carbonate) spray dryer for simultaneous removal of acid gases and trace contaminants such as mercury. The process requires cooling the flue gas by spray drying in the presence of large amounts of alkali sorbent material together with the activated carbon to enhance overall mercury removal.
U.S. Pat. No. 5,695,726 discloses a process in which toxic mercury vapor is removed from combustion gas by contact with dry alkaline material and dry activated carbon in a reaction chamber followed by solids separation. The adsorptive capacity of activated carbon decreases with increasing gas temperature. US ""726 emphasizes that a minimum level of HCl is necessary and a low temperature of the reaction chamber of from about 175xc2x0 C. to about 235xc2x0 C. are important for achieving high removal of the mercury. HCl is needed in the gas phase to react with elemental mercury or mercury oxide to convert them to chlorides. US ""726 also teaches that in the combustion of the wastes that are chlorine-deficient, an HCl-generating material such as scrap polyvinyl chloride plastic can be added to the chlorine-deficient waste prior to incineration. However, it is well known that adding chlorine to the waste stream and at the same time lowering the flue gas to the temperature range of 200xc2x0 C.-350xc2x0 C. are the two most favorable conditions for the synthesis or reformation of dioxins and furans.
U.S. Pat. No. 5,900,042 describes a process to remove elemental mercury from a gas stream by reacting the gas stream with an oxidizing solution to convert the elemental mercury to soluble mercury compounds. The gas stream is then passed through a wet scrubber to remove the mercuric compounds and oxidized constituents. The oxidizing solutions are solutions of aqueous iodine, aqueous bromine, aqueous chlorine, aqueous chloric acid and alkali metal chlorate and others.
U.S. Pat. No. 5,607,496 discloses a removal process, in which the elemental mercury of the combustion gas is first catalytically oxidized to form a mercury compound, and then the mercury compound is either adsorbed on adsorbent particles such as alumina or removed from the gas stream by scrubbing. The catalysts include mostly oxides of existing heavy metals in the combustion gas such as manganese, vanadium, lead, chromium, iron, cobalt, nickel and selenium.
UK Patent No. 1,336,084 discloses a process in which mercury vapour in the flue gas is removed by scrubbing the flue gas with a solution of alkaline earth metal hypochlorite containing an alkali metal chloride or alkaline earth metal chloride in excess of the chemical equivalent of the alkaline earth metal hypochlorite at a pH in the range of 8 to 12.
Nitrogen Oxides Removal:
Nitrogen oxides can be removed from combustor exhaust gas by selective catalytic reduction (SCR), selective non catalytic reduction (SNCR), and wet flue gas denitrification.
U.S. Pat. No. 4,220,632 discloses a process in which ammonia is used to reduce nitrogen oxides in combustion exhaust gas in the presence of a catalyst by SCR. High performance can be achieved with this technique, but it requires injection of ammonia into the exhaust gas prior to entering the SCR reactor. Sometimes it is necessary to first pass through a wet removal process to eliminate dust and poisonous chemicals that hinder the SCR process, then reheat the gas for the SCR. This method requires space due to the extent of the treatment equipment and generates a potential hazardous spent catalyst. Therefore, both capital and operating costs are high.
U.S. Pat. No. 3,900,554 describes a process called selective non-catalytic reduction (SNCR) in which ammonia is used to reduce nitrogen oxide from combustion effluents. Application of the technique is limited, because excessive unreacted ammonia or ammonia slip can not only add to the pollution, but also cause pluggage of the downstream equipment.
U.S. Pat. No. 4,719,092 describes another SNCR process but, instead of ammonia, urea is injected in the post combustion zone at a temperature between 850-950xc2x0 C. This reductant reagent is oxidized to ammonia, which then reacts with NOx to produce N2, water vapor and CO2. The technique claims better control of ammonia slip than the technique using ammonia. Maintaining a close temperature control is critical and difficult under this technique.
Since the majority of NOx in the off-gas is in the form NO, which has a very low solubility in water (k0H=0.0019 [mol/kg.bar] @ 298.15xc2x0 K.), it is difficult to reduce the amount of NOx in standard wet scrubbing pollution control equipment. However, if the NO can be oxidized to a higher state such as NO2 or NO3, and/or formed into another compound which has a higher solubility (NO2: k0H=0.01-0.04 [mol/kg.bar] @ 298.15xc2x0 K.; NO3: k0H=0.6-12.0 [mol/kg.bar] @ 298.15xc2x0 K.), then a larger amount of NOx can be removed.
U.S. Pat. No. 4,035,470 describes a process to remove both sulfur oxides and nitrogen oxides from the exhaust gas by adding ozone (O3) or chlorine dioxide (ClO2) to the exhaust gas and by scrubbing the exhaust gas with an aqueous scrubbing solution. O3 or ClO2 are good oxidants and are capable to convert NO in the gas phase to more soluble forms such as NO2 or N2O5. However, O3 is expensive and ClO2 is difficult to store and is hazardous.
U.S. Pat. No. 4,294,928 describes a liquid phase process using chlorine as oxidant in the presence of water in the scrubbing system. It has been claimed that the oxidation of nitric oxide to other oxides of nitrogen proceeds over a wide range of temperatures of the aqueous solution. A nitric oxide reduction of over 90% has been achieved at a temperature between 10xc2x0 C. and 50xc2x0 C.
JP 63-100,918 discloses a method of removing both mercury and nitrogen oxides from exhaust gas by washing the exhaust gas in a washing column using a solution comprising alkali and hypochlorite or chlorite.
EP 0 962,247 discloses a process of removing both NOx and SOx from a gaseous effluent by passing the gaseous effluent through an aqueous alkaline scrubber. The pH of the scrubber should be between 7 and 14, but is preferably very basic, i.e. between pH 10 and 14.
The present invention discloses an economical and simple method to remove both mercury and/or nitrogen oxides from combustion gas.
It has been found that calcium chloride added to the feed of the combustor promote the gas phase oxidation of elemental mercury to a more soluble form mercuric chloride, which can then be separated from the flue gas in a typical wet scrubber.
It has been found that calcium chloride added to the feed of the combustor promote gas phase and/or liquid phase oxidation of nitrogen monoxide which comprises the majority of flue gas NOx to a more soluble form of nitrogen oxides (NO2, NO3, N2O5 or others), which can be more easily scrubbed from the flue gas in a typical wet scrubber.
The process according to one aspect of the invention includes the following steps:
1. Introducing calcium chloride into the feed about to or undergoing incineration to facilitate formation in situ of hydrochloric acid in the flue gas generated by the sludge incinerator and reacting calcium with water contained in the feed at ambient temperature or at combustor operating temperature and with water in the wet scrubber to produce CaO and/or Ca(OH) 2:
CaCl2+H2Oxe2x86x922HCl+CaOxe2x80x83xe2x80x83(1)
CaCl2+H2Oxe2x86x922HCl+Ca(OH)2xe2x80x83xe2x80x83(2)
2. Converting gaseous Hg and HCl into HgCl2 by cooling Hg and HCl containing flue gas from typical operating temperature of 850xc2x0 C. to a temperature of about 450xc2x0 C.
Hg+2HCl+xc2xdO2xe2x86x92HgCl2+H2Oxe2x80x83xe2x80x83(3)
The mercury speciation of reaction (3) is favored by low temperature and occurs in the gas phase downstream of the combustor when the temperature starts to drop from tie typical 850xc2x0 C. to about 450xc2x0 C. A temperature lower than about 450xc2x0 C. is acceptable but not desirable, to avoid the temperature zone attributed to the formation of dioxins and furans. Furthermore, since reaction (3) is a rate limited reaction, adequate time is provided for speciation to occur within the favorable temperature window of 850xc2x0 C. and 450xc2x0 C.
3. Oxidizing nitrogen monoxide, which comprises the majority of flue gas NOx to a more soluble form of nitrogen oxides:
2NO+Ca(OH)2+xc2xdO2xe2x86x92Ca(NO2)2+H2Oxe2x80x83xe2x80x83(4)
4NO2+2Ca(OH)2xe2x86x92Ca(NO2)2+Ca(NO3)2+2H2Oxe2x80x83xe2x80x83(5)
Oxidation reactions (4) and (5) or other similar reaction can occur either in the gas phase or in the liquid phase of the wet scrubber. Excess calcium chloride fed to the Fluid Bed Combustor (FBC) or calcium oxide and calcium hydroxide generated from equations (1) and (2) increase the alkalinity and the pH of the wet scrubber water. A high pH of the scrubber water is favorable to the removal of both mercury and nitrogen oxides.
4. Quenching the soluble HgCl2 and NOx containing flue gas to about 70xc2x0 C.-90xc2x0 C. with water. The pollutants will be absorbed in the liquid phase and separated from the flue gas.
5. Cooling the flue gas to about 45-50xc2x0 C. with water to improve the absorption and the separation of the pollutants as described in step #4.
6. Further cooling the flue gas to ambient temperature or lower to condense and separate fugitive Hg from the flue gas.
In another aspect, the invention relates to a process for removing Hg from combustion flue gas generated by combustion of carbonaceous material. The steps include: introducing into the sludge a chlorine containing substance to facilitate formation of hydrochloric acid in the flue gas generated by the combustor; converting gaseous Hg and HCl into HgCl2 by cooling from combustor operating temperature Hg and HCl containing flue gas to a temperature of about 450xc2x0 C.; quenching the HgCl2 containing flue gas to about 70-90xc2x0 C.; separating Hg in the form of HgCl2 from the flue gas; cooling the flue gas in the presence of water to about 45-50xc2x0 C; removing residual HgCl2 and condensed water vapor from the flue gas; condensing fugitive elemental Hg by contacting the flue gas with further cooling water to reduce flue gas temperature to substantially ambient or lower temperature; and separating any condensed fugitive elemental Hg from the flue gas.
In another aspect, the invention relates to a process for reducing NOx emissions generated by combustion of carbonaceous material. The steps include: introducing into the carbonaceous material about to or undergoing combustion an alkali earth metal containing substance; reacting alkali earth metal (M) in the alkali earth metal containing substance with water contained in the feed at ambient temperature or at combustor operating temperature and with water in the wet scrubber to produce MO and/or M(OH)2; reacting MO and/or M(OH)2 with NOx in the combustor to produce M(NO2)2 and/or M(NO3)2; and separating water and M(NO2)2 and/or M(NO3)2 from flue gases generated by the combustor.
The invention and the advantages provided thereby will be more fully understood with the reference to the following detailed description of the preferred embodiment taken in conjunction with the accompanying drawings.